Background color:
Instead of letting bootstrap do the mapping between @state-warning-text and @alert-warning-text like I assumed every theme would do, Readable uses itsa own CSS to remap it to another variable: https://github.com/xwiki/xwiki-platform/blob/d8832778fec9f6ba8f4af438654f2d6420bd91c7/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-theme/xwiki-platform-flamingo-theme-bootswatch/src/main/resources/FlamingoThemes/Readable.xml#L1546-L1548

It's instead mapped to the border color @state-warning-border ...
Which is itself, rightfully so, mapped to the generic color @brand-warning which should be used for text or lines, not backgrounds.